Re: [PATCH 10/13] sched_ext: Hook up hardlockup detector

[Andrea Righi]

On Sun, Nov 09, 2025 at 08:31:09AM -1000, Tejun Heo wrote:
> A poorly behaving BPF scheduler can trigger hard lockup. For example, on a
> large system with many tasks pinned to different subsets of CPUs, if the BPF
> scheduler puts all tasks in a single DSQ and lets all CPUs at it, the DSQ lock
> can be contended to the point where hardlockup triggers. Unfortunately,
> hardlockup can be the first signal out of such situations, thus requiring
> hardlockup handling.
> 
> Hook scx_hardlockup() into the hardlockup detector to try kicking out the
> current scheduler in an attempt to recover the system to a good state. The
> handling strategy can delay watchdog taking its own action by one polling
> period; however, given that the only remediation for hardlockup is crash, this
> is likely an acceptable trade-off.
> 
> Reported-by: Dan Schatzberg <schatzberg.dan@xxxxxxxxx>
> Cc: Emil Tsalapatis <etsal@xxxxxxxx>
> Cc: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Makes sense to me, from a sched_ext perspective:

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  include/linux/sched/ext.h |  1 +
>  kernel/sched/ext.c        | 18 ++++++++++++++++++
>  kernel/watchdog.c         |  9 +++++++++
>  3 files changed, 28 insertions(+)
> 
> diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
> index e1502faf6241..12561a3fcee4 100644
> --- a/include/linux/sched/ext.h
> +++ b/include/linux/sched/ext.h
> @@ -223,6 +223,7 @@ struct sched_ext_entity {
>  void sched_ext_dead(struct task_struct *p);
>  void print_scx_info(const char *log_lvl, struct task_struct *p);
>  void scx_softlockup(u32 dur_s);
> +bool scx_hardlockup(void);
>  bool scx_rcu_cpu_stall(void);
>  
>  #else	/* !CONFIG_SCHED_CLASS_EXT */
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 4507bc4f0b5c..bd66178e5927 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3710,6 +3710,24 @@ void scx_softlockup(u32 dur_s)
>  			smp_processor_id(), dur_s);
>  }
>  
> +/**
> + * scx_hardlockup - sched_ext hardlockup handler
> + *
> + * A poorly behaving BPF scheduler can trigger hard lockup by e.g. putting
> + * numerous affinitized tasks in a single queue and directing all CPUs at it.
> + * Try kicking out the current scheduler in an attempt to recover the system to
> + * a good state before taking more drastic actions.
> + */
> +bool scx_hardlockup(void)
> +{
> +	if (!handle_lockup("hard lockup - CPU %d", smp_processor_id()))
> +		return false;
> +
> +	printk_deferred(KERN_ERR "sched_ext: Hard lockup - CPU %d, disabling BPF scheduler\n",
> +			smp_processor_id());
> +	return true;
> +}
> +
>  /**
>   * scx_bypass - [Un]bypass scx_ops and guarantee forward progress
>   * @bypass: true for bypass, false for unbypass
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 5b62d1002783..8dfac4a8f587 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -196,6 +196,15 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
>  #ifdef CONFIG_SYSFS
>  		++hardlockup_count;
>  #endif
> +		/*
> +		 * A poorly behaving BPF scheduler can trigger hard lockup by
> +		 * e.g. putting numerous affinitized tasks in a single queue and
> +		 * directing all CPUs at it. The following call can return true
> +		 * only once when sched_ext is enabled and will immediately
> +		 * abort the BPF scheduler and print out a warning message.
> +		 */
> +		if (scx_hardlockup())
> +			return;
>  
>  		/* Only print hardlockups once. */
>  		if (per_cpu(watchdog_hardlockup_warned, cpu))
> -- 
> 2.51.1
>